1. Filed of the Invention
The present invention relates to an image reading device and an image reading method of so called sheet-through type of conveying a subject document above one main surface of a transparent plate and reading an image of the document from other main surface side of the transparent plate.
2. Description of the Related Art
When an image of a document is read out by using an image reading device of a sheet-through type and printed with a printer or is taken into a computer such as a personal computer to be displayed on a display, there are some cases where a linear abnormal image appears on an image along a sub-scanning direction. This is normally caused by foreign substances such as dirt, dusts or paper powders adhered to a contact glass of an image reading device, or scars left on a contact glass. Such linear abnormal image appears on a white background portion as a black line or on an image portion as a white line, and is visually eye-catching. For an image device which is desired to obtain image data truly reflecting a document, it is desired to remove such linear abnormal image.
Technologies to suppress generating of such linear image are disclosed, for example, in Japanese Unexamined Patent Publication Nos. 2000-196814 and 2000-310820, respectively.
In the technology disclosed in the Japanese Unexamined Patent Publication No. 2000-196814, a reference plate arranged extendingly in a main scanning direction is read out at a document reading position before a document is read out. Then, it is determined whether or not an abnormal image is included in the obtained image of the reference plate. If an abnormal image is included, a document reading position is moved a predetermined distance along a sub-scanning direction. Further, if an abnormal image is identified again at a new document reading position, a document reading position is further changed. The technology disclosed in the Japanese Unexamined Patent Publication No. 2000-196814 suppresses appearing of a linear abnormal image of a document read out in such manner.
Further, in the technology disclosed in the Japanese Unexamined Patent Publication No. 2000-310820, at first, a document is read out, and it is determined whether or not there is included in the final line of a document an image having a width within a predetermined range and a density equal to or higher than a predetermined density. Secondly, when it is determined that such image is included, a reference plate is read out. Then, it is determined whether or not an image of the reference plate at the position of the above-described image has a density equal to or higher than the predetermined density. Thirdly, when it is determined that the image has a density which is equal to or higher than a predetermined density, it is determined whether or not an average density of images on opposite sides of the above-described position is equal to or higher than a predetermined density. Then, when it is determined that the average density is equal to or higher than the predetermined density according to the determination, it is determined that the image is an abnormal image. Further, if an abnormal image is identified again at a new document reading position, a document reading position is further changed.
Further, in the technology disclosed in the Japanese Unexamined Patent Publication No. 2000-310820, at first, a document is read out, and it is determined whether or not a difference between respective maximum density positions in the first line and final line of the document is within a predetermined range. Secondly, when the difference is within the predetermined range, it is determined whether or not the maximum density in the final line is equal to or higher than a predetermined density. Thirdly, when the maximum density is equal to or higher than the predetermined density according to the determination, the reference plate is read out, and it is determined whether or not the maximum density of the reference plate is equal to or higher than a predetermined density. Fourthly, when the maximum density is equal to or higher than the predetermined density according to the determination, it is determined whether or not a difference between the maximum density position in the final line read out in the first step and the maximum density position of the reference plate is within a predetermined range. Then, when it is determined that the difference is within the predetermined range, it is determined that there is an abnormal image at the maximum density position. Further, if an abnormal image is identified again at a new document reading position, a document reading position is further changed.
The technology disclosed in the Japanese Unexamined Patent Publication No. 2000-310820 performs determinations successively as described above to determine foreign substances adhered to a reference plate and scars left on the reference plate which are not appeared as an abnormal image on actual image data, thereby improving an accuracy in determining foreign substances adhered to the contact glass and scars left on the contact glass.
According to the disclosures of Japanese Unexamined Patent Publication No. 2000-310820, a document reading position is changed successively from the initial document reading position to the final document reading position which are set in advance at each time when an abnormal image is identified as described above. However, when a document reading position reaches the final document reading position, it is changed in a reversed order (from the final document reading position to the initial document reading position determined in advance). At that time, a warning is given to a user to notify that it is the time for maintenance.
There is a case where dirt, dusts, paper powders and the like causing the above-described abnormal images are moved when a document passes the document reading position, and a linear abnormal image may not be generated on an image even when it is determined that there is an abnormal image. If the above-described warning is given in such a case, the warning may likely to become obtrusive, or a user may feel it burdensome if the image reading device is of a model which requires a user to perform an operation to cancel the warning. Accordingly, convenience of the image reading device is lowered. Further, if the warning is given in such a case, a user may think that the image reading device went wrong and is happened to perform an irrelevant operation. Especially when an image reading device reads a document which is likely to generate paper powders, it is likely that such warning is given frequently. Accordingly, such disadvantage is significant.